The present invention relates to a heat pump system in which the opening of an expansion valve is controlled depending upon the temperatures of each medium undergoing heat exchange at utilization side and non-utilization side heat exchangers.
First, a heat pump system of a conventional type will be described. In FIG. 1, the conventional unit includes a compressor 1, a four-way valve 2, a non-utilization side heat exchanger 3 serving as a condenser for cooling and as an evaporator for heating, a fan 4 for supplying a flow of ambient air to the non-utilization side heat exchanger 3, an expansion valve 5 of the temperature type, a temperature sensor 6 attached to the inlet piping 7 of the compressor 1, a pressure equalizer 8 of the expansion valve 5 connected to the inlet piping 7, a utilization side heat exchanger 9 serving as an evaporator for cooling and as a condenser for heating, and an accumulator 10.
The operation of this system during cooling will now be described. As indicated by solid-line arrows in FIG. 1, the refrigerant gas discharged from the compressor 1 flows to the non-utilization side heat exchanger 3 through the four-way valve 2 where it exchanges heat with air supplied by the fan 4 and is thereby condensed. The condensed refrigerant then flows to the utilization side heat exchanger 9 passing through a first check valve 21, the expansion valve 5 where its pressure is reduced, and a second check valve 22. In the utilization side heat exchanger 9, the refrigerant exchanges heat with water flowing in the heat exchanger 9, thereby cooling the water. The cooled water is then used to cool a room or rooms through a fan coil unit (not shown), etc. The refrigerant, after being evaporated in the utilization side heat exchanger 9 due to heat exchange with the water, returns to the compressor 1 through the four-way valve 2 and the accumulator 10.
Next, the operation of the system during heating will be described. As indicated by dotted-line arrows, the refrigerant gas discharged from the compressor 1 flows through the four-way valve 2 to the utilization side heat exchanger 9 where it exchanges its heat with the water flowing in the heat exchanger 9 to thus heat the water. The heated water is circulated in the room to heat the room through the fan coil unit in a manner similar to that used for air conditioning. The refrigerant is condensed in the utilization side heat exchanger 9 due to heat exchange with the water. Then, it is passed to the non-utilization side heat exchanger 3 through a third check valve 23, the expansion valve 5 where its pressure is reduced, and a fourth check valve 24. In the non-utilization side heat exchanger 3, the refrigerant is evaporated due to heat exchange with the air supplied by the fan 4, and then returned to the compressor 1 through the four-way valve 2 and the accumulator 10.
In the above-discussed system, the amount of opening of the expansion valve 5 is determined so as to control the flow of refrigerant in dependence upon the temperature difference, or amount of superheating, between the temperature of the refrigerant in the inlet piping 7 of the compressor 1 and the saturation temperature at the refrigerant pressure. Consequently, the degree of opening is governed solely by the conditions at the low pressure side, with substantially no response to changes in the conditions on the high pressure side. With the construction of a conventional heat pump unit described above, if conditions should change suddenly, for instance, due to a shower while operating in the cooling mode in the summer, the non-utilization side heat exchanger 3 will be cooled rapidly and, consequently, the pressure on the high pressure side lowered. However, the amount of opening of the expansion valve 5 is kept constant. Therefore, the flow rate of the circulating refrigerant decreases due to the reduced pressure difference between the high and low pressures, and also the pressure on the low pressure side drops, resulting in a reduction of cooling capacity.
In the heating mode, particularly during starting of the system, the utilization side heat exchanger 9 is cooled due to the low temperature of the circulating water, and hence the pressure on the high pressure side is low. Therefore, as in the case of cooling mentioned above, the pressure on the low pressure side drops, and the evaporation temperature of the non-utilization side heat exchanger 3 is reduced, causing frosting on the non-utilization side heat exchanger 3. As a result, frequent removal of frost is required, and the temperature of the water in the utilization side heat exchanger 9 cannot rise rapidly.